555 timer calculator for astable and monostable circuits
The 555 is the most-used timer IC ever made, and this 555 timer calculator covers its two classic circuits. In astable mode it turns R1, R2 and C into the output frequency, period, duty cycle and the separate high and low times; in monostable mode it turns R and C into the width of the single output pulse. Both modes also work backwards: give a target frequency and duty cycle (or pulse width) and the calculator finds the component values, then snaps them to E24/E12 standard parts and tells you the frequency you will actually get.
A schematic drawn for your values shows where each part connects (pin numbers included), and a waveform plot shows the pin 3 output with the capacitor voltage ramping between ⅓ and ⅔ VCC. Formulas use the exact ln 2 and ln 3 constants, which differ from the 1.44 and 1.1 datasheet approximations by well under 1 %.
How to use the 555 Timer Calculator
- Choose Astable (square wave) or Monostable (one pulse per trigger).
- Pick what to calculate: the output from your components, or the components for a target frequency and duty cycle (astable) or pulse width (monostable).
- Enter values with units — 1k, 4.7k, 100k for resistors; 10u, 100n, 470p for capacitors; 1k or 50 for hertz; 1, 500m or 10u for seconds.
- Read the frequency, period, duty cycle and high/low times (or the pulse width), the nearest standard values and any warnings.
- Check the schematic and waveform, then press “Copy summary” to keep the numbers.
555 Timer Calculator features
Astable in both directions
R1, R2, C → f, T, D, t_high, t_low; or f and D → R1, R2 (given C); or f → C (given R1, R2). Duty cycles of 50 % or less switch to the diode-across-R2 variant automatically.
Monostable both ways
R, C → T = 1.1·R·C, or T → R or C.
Standard-value snapping
Results are rounded to E24, E12 or E96 resistors and E12/E6 capacitors, and the frequency or pulse width with those real parts is shown next to the ideal one.
Schematic and waveform
An SVG circuit with pin numbers and component values, plus the output and capacitor-voltage waveforms scaled to your high and low times.
Practical warnings
Flags R1 under 1 kΩ, very large resistances, frequencies beyond the bipolar 555 and capacitors small enough to be dominated by stray capacitance.
555 astable formulas and a worked example
In the astable circuit the capacitor charges through R1 + R2 from ⅓ VCC to ⅔ VCC while the output is high, then discharges through R2 alone (into pin 7) back to ⅓ VCC while the output is low. Each leg of an RC charge between those thresholds takes ln 2 × RC, so t_high = 0.693·(R1 + R2)·C, t_low = 0.693·R2·C, the period is 0.693·(R1 + 2R2)·C and the frequency is f = 1.44 / ((R1 + 2R2)·C). The duty cycle D = (R1 + R2)/(R1 + 2R2) is therefore always above 50 % in the basic circuit; it approaches 50 % when R2 ≫ R1.
Example: R1 = 1 kΩ, R2 = 10 kΩ, C = 10 µF. R1 + 2R2 = 21 kΩ, so f = 1.4427 / (21 000 × 10⁻⁵) = 6.87 Hz (6.86 Hz with the 1.44 approximation), period 145.6 ms, t_high = 0.693 × 11 kΩ × 10 µF = 76.2 ms, t_low = 69.3 ms and D = 11/21 = 52.4 %.
Monostable formula
In the one-shot circuit a falling edge on pin 2 (below ⅓ VCC) sets the output high and lets the capacitor charge from 0 V through R. When it reaches ⅔ VCC the output goes low and pin 7 discharges the capacitor. Charging from 0 to ⅔ of the supply takes ln 3 × RC, hence T = 1.1·R·C: 100 kΩ and 10 µF give a 1.1 s pulse. The trigger pulse must be shorter than T (or be differentiated with a small RC), and the reset pin should be tied to VCC when unused.
| Want | Use | Example |
|---|---|---|
| Long delays (seconds–minutes) | Large R (up to ~1 MΩ) with a low-leakage capacitor | 1 MΩ + 100 µF ≈ 110 s |
| Audio tones (100 Hz–10 kHz) | R in the 1–100 kΩ range with 10–100 nF | 4.7 kΩ, 10 kΩ, 100 nF → 584 Hz |
| Fast clocks (>100 kHz) | CMOS 555 (TLC555, LMC555), small C ≥ 500 pF | 1 kΩ, 1 kΩ, 1 nF → 481 kHz |
| Exactly 50 % duty | Diode across R2 with R1 = R2, or divide a 2× clock with a flip-flop | D = 50 % with 10 kΩ / 10 kΩ + diode |
Design tips
- Keep R1 ≥ 1 kΩ: when the output is low, pin 7 shorts R1 to ground and must sink VCC/R1.
- Put a 10 nF capacitor from pin 5 (control voltage) to ground and a 100 nF decoupling capacitor across the supply.
- Electrolytic capacitors have poor tolerance and leakage; for timing accuracy use film or C0G ceramic parts, and check their codes with the capacitor code calculator.
- Identify resistor values with the resistor colour code calculator, and use the Ohm’s law calculator to size the load on pin 3 (up to about 200 mA on a bipolar 555).
555 Timer Calculator FAQ
How do I calculate the frequency of a 555 astable?
f = 1.44 / ((R1 + 2·R2) × C), with R in ohms and C in farads. Enter R1, R2 and C in the 555 timer calculator to get the frequency, period, duty cycle and the high and low times.
Why can’t the basic 555 astable go below 50 % duty cycle?
The capacitor charges through R1 + R2 but discharges through R2 only, so the high time is always longer than the low time. For 50 % or less, add a diode across R2 (anode to pin 7) so charging bypasses R2; the calculator switches to that variant automatically.
What is the 555 monostable formula?
T = 1.1 × R × C. With R = 100 kΩ and C = 10 µF the output stays high for 1.1 s after each trigger pulse on pin 2.
Which standard values does the calculator snap to?
E24 resistors and E12 capacitors by default; E12/E6 or E96/E12 can be selected. The result with those real parts is shown alongside the ideal one.
How accurate is a 555 timer?
The IC itself is within about 1 % and drifts around 50 ppm/°C, so accuracy is set by the components: ±5 % resistors and ±10 % capacitors can give ±15 % error. Use 1 % resistors and film or C0G capacitors, or add a trimmer, when it matters.